Slabs or blooms produced in continuous casting plants are used as the starting material for many rolling mill products, for example, slabs or thin slabs generated by such plants can be used for producing sheets or strips. To reduce segregation in the strand during continuous casting and to obtain better material texture, the strand thickness is generally reduced between 0.5 mm and 3 mm per meter in a soft-reduction line in the final solidification area of the continuous casting plant.
The soft-reduction line is generally of the type in which pairs of rollers can be adjusted individually or in units in relation to one another by means of hydraulic cylinders which apply a pressure to force the pairs of rollers towards each other. The gap between the rollers can be set continuously by means of adjustable spindles which mechanically determine the minimum gap between the rollers. When slabs or blooms are cast continuously, for example, in an arc continuous casting plant, the gap is set according to the strand shrinkage behavior over the length of the machine. In a typical soft-reduction line, the gap of pairs of rollers, individually or combined in units, is gradually reduced along the direction of strand movement depending on the shrinkage behavior of the strand, in order to improve inner quality of the strand material in the area of the final solidification. In this area, bending of the crossbeams and rollers are kept small over the width of the strand by using split rollers, for example.
Each roller of a roller pair is mounted on an upper yoke and the other roller of the roller pair is mounted on a lower yoke which yokes are pulled toward each other by the aforementioned hydraulic cylinders. The yokes can be adjusted relative to one another to change the gap between the rollers by means of rigid, adjustable length spindles which mechanically contact each yoke. With rollers mounted in units, several rollers may be mounted on each yoke and the yokes may be inclined from the inlet toward the outlet, so that the gap on the outlet side is smaller than it is on the inlet side, thereby causing the desired reduction in strand thickness as the strand passes through the roller unit.
In order to improve the strand texture, the strand should run through the soft-reduction area so that the desired reduction in thickness is achieved with a residual liquid phase remaining in the center of the strand during the reduction. Further, the length of the soft-reduction line should be adjusted so that the strand is solidified at the end of the soft-reduction area. In particular, soft reduction does nothing to improve the inner texture of the cast strand and there is no compression of the core texture if the strand is already solidified before it goes into the soft-reduction line or is not solidified by the time it reaches the end in the soft-reduction area.
Due to changing casting parameters, such as, especially, the casting speed, the temperature of the steel, the quality of the steel and secondary cooling, it is often difficult to set the reduction in the soft-reduction area so that the foregoing is achieved. This difficulty occurs because, in a conventional soft-reduction area, the gap must be set during a break in the casting and the gap can be varied to correspond to only a narrow range in casting speed.
It has therefore already been proposed that the gap be adjusted during the casting operation by adjusting the roller spindles depending on the actual casting parameters. This solution has the disadvantage that the spindles and the spindle drive elements used to make the adjustment must be set under load. If a solidified slab is passing between the rollers as the adjustment is being made, the force on the spindles can correspond to the hydraulic cylinder force and therefore, the force needed to make the spindle adjustment can be quite large. In addition, in the case where the strand has not solidified completely upon passing through the rollers, the gap must be set by determining the position of the tip of the liquid core and this position must be found by computer or by other measurement techniques.
Another suggestion to solve the problem is to set the gap continuously via hydraulic cylinders with built-in inductive position-measuring and servo-valves. This solution has the disadvantage that the apparatus is costly and requires a high expense for maintenance. As with the previous prior art approach, the position of the tip of the liquid core must be found by computer or by other measurement techniques in order to set the gap.
Therefore, an object of the invention is to create a process and a device that uses simple means to make it possible to set the gap, even during the casting operation, i.e., under load, especially while adapting to a changing situation in the soft-reduction area due to changing casting parameters.